Meaning of Android Studio error: Not annotated parameter overrides @NonNull parameter

Question

I\'m trying out Android Studio. Upon creating a new project and adding a default onSaveInstanceState method to the create MyActivity class, when I try to commit

Answer 1

In addition to the other answers, the @NonNull (and it's opponent, @Nullable) annotation annotates a field, parameter or method return type. IntelliJ and thus Android Studio can warn you for possible NullPointerExceptions at compile time.

An example is best here:

@NonNull private String myString = "Hello";

@Nullable private String myOtherString = null;

@NonNull 
public Object doStuff() {
    System.out.println(myString.length); // No warning
    System.out.println(doSomething(myString).length); // Warning, the result might be null.

    doSomething(myOtherString); // Warning, myOtherString might be null.

    return myOtherString; // Warning, myOtherString might be null.
}

@Nullable
private String doSomething(@NonNull String a) {
    return a.length > 1 ? null : a; // No warning
}

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These annotations do not alter runtime behavior (although I have experimented with this), but serve as a tool for preventing bugs.

Note that the message you received was not an error, but just a warning, which is safe to ignore, if you choose to. The alternative is to annotate the parameter yourself as well, as Android Studio suggests:

@Override
protected void onSaveInstanceState(@NonNull Bundle outState) {
    super.onSaveInstanceState(outState);
}